The present invention relates to electrodes and the use of gas-diffusion electrodes in a variety of applications. The present invention further relates to methods of preparing gas-diffusion electrodes, including the carbon supports for gas diffusion electrodes. The present invention also relates to materials particularly suitable in the manufacture of improved gas-diffusion electrodes, such as air diffusion electrodes.
With respect to gas-diffusion electrode structures, multi-layered composite electrodes are the preferred solution. Depending on the final use of the electrode, two or more layers of carbon blacks combined with fluorine containing compounds are joined into a continuous structure. Double-layered electrodes have a highly hydrophobic carbon support layer coupled to a lesser hydrophobic layer (also known as the active layer) containing catalyzed carbon and suitable binders. A metal screen embedded in the carbon serves as the current collector. The hydrophobic part of the electrode contains a gas feed channel so that the reactant gas can easily diffuse through the pores towards the electroactive layer, where reactions take place. This part of the electrode acts as a barrier to prevent penetration of the electrolyte. Electrolyte in the pores would prevent the diffusion of gas to the reaction layer and this would result in a dramatic deterioration of the electrode's performance. As stated before, the active layer is less hydrophobic than the diffusion layer to ensure partial wetting of the carbon and the electrocatalyst particles. In the active layer, which is also known as the catalyst layer, the reactant gas supplied from the blocking layer diffuses in the gas channels to be dissolved in an electrolyte in contact with carbon or catalyzed carbon so that the electrode reaction is carried out on the carbon or catalyzed carbon in the electrolyte. The structure and the hydrophobic properties of the active layer can be important for efficient electrode operation. It is generally recognised that the major concern in developing an efficient electrode is to improve the wettability of the active layer.
In a three-phase reaction system, such as the active layer, a stable interface between the electrolyte and the gas has to be maintained so that the number of reaction sites remains as high as possible for long operation times. Regarding this point, the ratio of liquid and gas pores in the active layer determines the mass transfer conditions. Poorly wetted pores will result in an acceptably high electrical resistivity and will have low catalyst utilization due to lack of electrolyte, whereas a more hydrophilic interface may flood. Pores with an optimal wettability are filled with only a small film of electrolyte so that the gas diffusion limitations are significantly reduced. The electrolyte quantity in the active layer can be adjusted by a change in the fluoropolymer content in the active layer.
A great variety of wet proofed gas diffusion electrodes exist at the present time which differ in overall structure and configuration. A gas diffusion electrode is generally produced by mixing conductive carbon fine powder and the hydrophobic/hydrophilic fluorine resin powder or suspension thereof, forming the mixture into a sheet, and sintering the sheet.
Water-repellent structures of the diffusion layer are generally achieved by coating the surface of some carbon particles with a hydrophobic material. Polytetrafluoroethylene (PTFE) is one of the most stable and effective hydrophobic agent known. The most popular of the PTFE materials used is in the form of a colloidal suspension, produced by Du Pont de Nemours and Co, Inc. under their Teflon.RTM. trademark (Teflon 30-N). The incorporation of PTFE in the blocking layer serves two functions: binding the high surface carbon particles into a cohesive structure and imparting hydrophobicity to the layer.
The most common method to make carbon more hydrophobic is a wet application method. A colloidal aqueous PTFE suspension is blended with carbon powder in an alcohol/water solution to give a mixture containing 5-60 wt.% Teflon.RTM.. This mixture is normally produced in the form of an aqueous paste, and it can be rolled, spread, printed, or sprayed onto a substrate, for example a carbon paper. For example, U.S. Pat. No. 5,531,883, incorporated in its entirety by reference herein, relates a method for preparing a hydrophobic support layer by dispersing carbon black in water and adding an aqueous dispersion of PTFE.
U.S. Pat. No. 5,561,000, also incorporated in its entirety by reference herein, relates to a process in which a mixture of carbon and a PTFE suspension is filtered and the filtered-off paste is spread out on a carbon sheet which has been previously soaked in a hydrophobic rendering material such as PTFE in a suspension. The filtered-off paste is applied and pressed in the carbon support by means of a scraping knife. Some cathode structures utilise layers of polytetrafluoroethylene to form protective or backing sheets in order to further increase the hydrophobicity of the carbon black cathodes on the air side.
In the active layer or catalytic layer, a semi-hydrophobic structure is preferred for a more efficient use of the catalyst, and consequently hydrophilic ingredients are used in the air electrode preparation.
The most common method to make carbon partly hydrophilic consists in preparing an alcohol mixture of the carbon powder (with or without catalyst) and a hydrophilic fluorinated resin. One of the most popular hydrophilic fluorinated polymers available on the market is a perfluoric sulphonic acid polymer produced by Du Pont de Nemours and Co, Inc. under their Nafion.RTM. trademark (Nafion solution SE-5112). For example, in U.S. Pat. No. 4,877,694, also incorporated in its entirety by reference herein, a mixture of finely powdered active material is prepared by blending catalysed carbon particles together with an alcohol solution of Nafion..RTM. The resulting mixture is dried and finely chopped. An alcohol dispersion of the above product is then filtered on a first prepared backing layer to form a dual phase sheet which is dried and sintered under pressure.
Several further techniques have been developed to increase the catalyst utilization. According to a number of techniques, the catalyst is applied directly on a solid electrolyte membrane and not on the electrode. For example, U.S. Pat. No. 5,561,000 relates to a method for making a gas diffusion layer wherein a catalytic layer is formed on a porous hydrophobic back support in the form of a liquid ink prepared by mixing catalyst particles (20% Pt/C) and a proton conductive monomer solution, such as 5% solution of Nafion.RTM.. A non-catalytic intermediate layer containing a mixture of an electron conductive material, such as carbon, and the proton conductive monomer is provided between the support and the catalytic layer. In some cathode structures the solution is made of Pt/C catalyst powder, a Nafion.RTM. solution, PTFE in suspension, and carbon black and it is applied directly on a Nafion.RTM. membrane.
The problem with these methods is the difficulty in simultaneously obtaining the required porosity and firmness of the layers provided on the support. All the above-mentioned patents relate to the use of carbon combined with a colloidal mix, dry mix, or fluorinated polymer solutions. All of the proposed processes involve coating carbon black particles with a fluorinated polymer compound. Although the above-mentioned literature may provide methods which may provide carbon with the proper hydrophobicity/hydrophilicity balance, the methods require elaborate and complex steps or require relatively expensive raw materials.